RIYADH (Reuters) - U.S. President Barack Obama told Saudi king Abdullah he would not agree to a bad deal with Iran on its nuclear program, a senior American official said, on a visit aimed at allaying the kingdom's concerns that their decades-old alliance is faltering.

While the two leaders discussed "tactical differences", they both agreed their strategic interests were aligned, the official said. A White House statement after the two hours of talks said Obama had reiterated the significance Washington placed on its "strong" ties with the world's largest oil exporter.

"I think it was important to have the chance to come look him (king Abdullah) in the eye and explain how determined the president is to stop Iran from getting a nuclear weapon," the official said.

The meeting was a chance to assure the king that "we won't accept a bad deal and that the focus on the nuclear issue doesn't mean we are not concerned about, or very much focused on, Iran's destabilizing activities in the region."

The official said the two had had a full discussion about Syria, where a three-year-old civil war has killed an estimated 140,000 people and uprooted millions. The two nations were working together "very well" to bring about political transition and support moderate opposition groups, the official said.

Saudi officials made no immediate comment on the meeting but Saudi state media said the talks were focused on Middle East peace efforts and the Syrian crisis.

Last year senior Saudi officials warned of a "major shift" away from Washington after bitter disagreements about its response to the "Arab spring" uprisings, and policy towards Iran and Syria, where Riyadh wants more American support for rebels.

REASSURANCE

However the White House statement said the two countries were cooperating to address issues including Syria, Iran, combating extremism and supporting Middle East peace talks.

The elderly king, accompanied by a number of senior princes, had what appeared to be an oxygen tube connected to his nose at the start of the meeting at his desert farm at Rawdat Khuraim northeast of the capital Riyadh, witnesses said.

Saudi state television showed Obama, accompanied by U.S. Secretary of State John Kerry and National Security Adviser Susan Rice, listening attentively while King Abdullah spoke, gesticulating with both hands as he made a point.

The Saudis want more reassurance on American intentions regarding talks over Iran's nuclear program, which might eventually lead to a deal that ends sanctions on Tehran in exchange for concessions on its atomic facilities.

Riyadh fears such a deal could come at the expense of Sunni Arabs in the Middle East, some of whom fear that Shi'ite Iran will take advantage of any reduction in international pressure to spread its influence by supporting co-religionists.

Major powers suspect Iran's nuclear program is aimed at developing a nuclear weapons capability. Tehran said its work is aimed only at generating electricity.

The senior official said the two had not had time to discuss the kingdom's human rights record.

In the run-up to the visit, officials had said Obama would aim to persuade the monarch that Saudi concerns that Washington was slowly disengaging from the Middle East and no longer listening to its old ally were unfounded.

AID FOR REBELS

Overwhelmingly Sunni Muslim Saudi Arabia is backing the insurgents in their battle to oust Syrian President Bashar al-Assad, who is supported by Riyadh's rival, Shi'ite power Iran.

U.S. Deputy National Security Adviser Ben Rhodes said coordination with the kingdom on Syria policy, particularly regarding providing help to the Syrian rebels, had improved.

"That's part of the reason why I think our relationship with the Saudis is in a stronger place today than it was in the fall when we had some tactical differences about our Syria policy," he told reporters on Air Force One.

One area where Riyadh has long differed from Washington is in Obama's reluctance to supply rebels with surface-to-air missiles, sometimes known as MANPADS.

The Washington Post reported on Friday that the U.S. was ready to increase covert aid to Syrian rebels under a new plan which included training efforts by the CIA, and was considering supplying MANPADS.

The White House has not closed the door to the possibility of such a move in the future, but the senior official said the U.S. remained concerned about providing such weapons to rebels.

Obama has shown himself wary of being drawn into another conflict in the Muslim world after working hard to end or reduce American military involvement in Iraq and Afghanistan.

While Saudi Arabia supplies less petroleum to the United States than in the past, safeguarding its energy output remains important to Washington, as does its cooperation in combating al Qaeda.

FEARS OVER IRAN

An editorial in the semi-official al-Riyadh newspaper on Friday said Obama did not know Iran as well as the Saudis, and could not "convince us that Iran will be peaceful".

"Our security comes first and no one can argue with us about it," it concluded.

The Saudi king was accompanied in the talks by Crown Prince Salman, Prince Muqrin, who was named second-in-line to rule on Thursday, and Foreign Minister Prince Saud al-Faisal.

Powerful Interior Minister Prince Mohammed bin Nayef, who recently met top U.S. officials in Washington to discuss Syria, was not present.

Also present was the new American ambassador in Riyadh, Joseph Westphal, whose appointment was confirmed by the Senate late on Wednesday, apparently in order to let him attend Friday's meeting.